Primitive whales evolved during the early Eocene period, at least 53 - 54 million years ago. Fossil remains indicate that whales evolved from hoofed land mammals - perhaps the shore-dwelling, hyena-like Mesonychid that started a returned, bit by bit, to the sea roughly 50 million years ago.

Another possible step in whale ancestry is the otter-like Ambulocetus, an extinct mammal the size of a sea lion, 10 feet (3 m) long and about 650 pounds. Its limbs allowed it to swim and could also support it on land. It had long, powerful jaws with shark-like teeth, a small brains, and a pelvis fused to its backbone (like land-dwelling mammals but unlike whales).

Himalayecetus subathuensis is the oldest-known whale, about 53.5 million years old. Its fossilized jawbone and teeth were found in the Simla Hills of northern India, in the foothills of the Himalayan mountains. This area was underwater (in the Tethys sea) during the Tertiary period (when India was an island off Asia). This whale may have spent some of its time on land.

The later Pakicetus, a primitive whale with pointed teeth like Mesonychid and a pinched brain case like Ambulocetus, had a water-adapted inner ear but still had four limbs. It had nostrils located at the front of its head; it had no blowhole!

Rodhocetus, even more whale-like, had four short limbs (the femur or thigh bone was only 8 inches - 20 cm long) but had a whale-like body and its pelvis was no longer fused to the backbone. It had a long snout with many teeth.

Dorudon atrox, another early whale, was about 20 feet (6 m) long. Its arms were small, flattened flippers. The rear limbs were only about 4 inches (10 cm) long. It also had a long snout with many teeth.

Basilosaurus, a very primitive, extinct whale, had a tiny head and pointed snout with teeth, unlike modern-day whales which have large heads and a blunter snout. It was about 65 ft (20 m) long.

Baleen whales evolved from the toothed whales during the Oligocene Epoch, roughly 30 million years ago. The earliest-known baleen whale was Aetiocetus, which still had all its teeth, but it also had a loose jaw hinge like later baleen whales.

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